To rewrite \( 15^{-38} \) as an expression with positive exponents only, we can apply the property of negative integer exponents, which states that \( a^{-n} = \frac{1}{a^{n}} \).
So, for \( 15^{-38} \):
\[ 15^{-38} = \frac{1}{15^{38}} \]
Thus, the expression with positive exponents is:
\[ \frac{1}{15^{38}} \]